Automated External Defibrillators include signal processing software that analyzes ECG signals acquired from the victim to determine when a cardiac arrhythmia such as Ventricular Fibrillation (VF) or shockable ventricular tachycardia (VT) exists. Usually, these algorithms are designed to perform ECG analyses at specific times during the rescue event. The first ECG analysis is usually initiated within a few seconds following attachment of the defibrillation electrodes to the patient. Subsequent ECG analyses may or may not be initiated based upon the results of the first analysis. Typically if the first analysis detects a shockable rhythm, the rescuer is advised to deliver a defibrillation shock. Following the shock delivery a second analysis is automatically initiated to determine whether the defibrillation treatment was successful or not (i.e. the shockable ECG rhythm has been converted to a normal or other non-shockable rhythm). If this second analysis detects the continuing presence of a shockable arrhythmia, the AED advises the user to deliver a second defibrillation treatment. A third ECG analysis may then be initiated to determine whether the second shock was or was not effective. If a shockable rhythm persists, the rescuer is then advised to deliver a third defibrillation treatment.
Following the third defibrillator shock or when any of the analyses described above detects a non-shockable rhythm, treatment protocols recommended by the American Heart Association and European Resuscitation Council require the rescuer to check the patient's pulse or to evaluate the patient for signs of circulation. If no pulse or signs of circulation are present, the rescuer is trained to perform CPR on the victim for a period of one or more minutes. Following this period of cardiopulmonary resuscitation (that includes rescue breathing and chest compressions) the AED reinitiates a series of up to three additional ECG analyses interspersed with appropriate defibrillation treatments as described above. The sequence of 3 ECG analyses/defibrillation shocks followed by 1-3 minutes of CPR, continues in a repetitive fashion for as long as the AED's power is turned on and the patient is connected to the AED device. Typically, the AED provides audio prompts to inform the rescuer when analyses are about to begin, what the analysis results were, and when to start and stop the delivery of CPR.
One limitation associated with many AEDs is that the period between each set of ECG analyses and shocks is pre-programmed into the device and is fixed for all rescue situations. When the application of CPR due to lack of circulation is warranted, this pre-programmed period is consumed by the delivery of rescue breaths and chest compressions. When no CPR is warranted because the last shock was effective in converting the patient to a perfusing cardiac rhythm, this pre-programmed period is consumed by periodically monitoring the patient's pulse and assuring that no relapse or re-fibrillation has occurred. Under some out-of-hospital rescue protocols, the period between successive sets of ECG analyses can be as long as 3 minutes.
It is commonly known that victims of cardiac arrest who have been successfully defibrillated sometimes relapse into ventricular fibrillation shortly after a successful shock treatment. In such cases, the rescuer who verified the presence of a pulse immediately following defibrillation and thus decided not to perform CPR, may be unaware that the victim's condition has deteriorated until the next ECG analysis is performed some 1-3 minutes later. Under these conditions, the delivery of needed defibrillation treatments and CPR may be delayed.
Some AEDs are designed to avoid this undesirable delay in treatment by continuously and automatically analyzing the victim's ECG whenever defibrillation electrodes are connected to the patient. These AEDs perform a continuous “background” analysis that evaluates the victim's ECG signals during the 1-3 minute CPR/monitoring period between analysis/shock sequences. As such, they are able to detect refibrillation of the victim's heart (should it occur) and promptly advise the rescuer of the patient's deteriorated condition. While these “improved” systems help prevent the delay in treatment that can result from undetected refibrillation of the patient's heart, they are also susceptible to misinterpreting the ECG artifact introduced by CPR related chest compressions as shockable arrhythmias. When the ECG analysis algorithm misinterprets CPR related ECG artifact as a shockable rhythm, it may advise the rescuer to prematurely stop performing CPR and to deliver a defibrillation treatment. While in some cases immediate defibrillation may be the appropriate treatment, some clinical research suggests that an appropriately long period of CPR between sequences of defibrillation treatments may be more beneficial to the patient than immediate defibrillation, particularly when VF has been of long duration or persistently recurring. Furthermore when the victim's cardiac rhythm is not treatable by defibrillation therapy (non-shockable) but incompatible with life such as in cases of asystole or electromechanical disassociation, the premature cessation of CPR in response to the erroneous detection of a shockable cardiac rhythm can reduce the patient's chances of survival.
For those AEDs that perform background ECG analysis during periods between analysis/shock sequences, a common strategy for ensuring that sufficient time is provided for effective CPR delivery even in the presence of shockable rhythms is to disable background ECG analysis or ignore the results of this analysis for a predetermined time period following the last shock in each treatment sequence. During this period, the rescuer is allowed to perform CPR without advice from the unit that a shockable rhythm is present. Following this period, ECG analysis results are used to prompt the user to stop CPR and thus allow a CPR artifact free ECG analysis to be performed.
Since most currently available AEDs are not equipped to detect or monitor the delivery of CPR related chest compressions, they are incapable of determining when CPR artifact is present in the ECG signals and when it is not. The automatic activation/deactivation of their background ECG analysis function, therefore, is based exclusively upon time since the last shock or completion of the last “foreground” ECG analysis. If the delivery of CPR is stopped during this period when background ECG analyses have been disabled, life threatening changes in the patient's ECG rhythm will remain undetected (even though it could be effectively analyzed) for at least some period of time during the rescue event.